The present invention is related to devices for lifting large objects, and more specifically to portable jacks adaptable for lifting bulky objects from a position near the ground to some distance above the ground.
Many types of lifting jacks have been developed in the past for lifting heavy objects including screw jacks, screw-type scissor jacks, mechanical pawl and ratchet jacks, hydraulic cylinder jacks, air cylinder jacks, and modifications of these jacks using various mechanisms and configurations of levers for adaption to specific purposes. Since the working principle of most jacks is to lift from the underside of bulky objects, some amount of clearance is required to accommodate the mechanism of the jack. Various attempts have been made to overcome or at least minimize this clearance requirement, usually by the use of some lever arrangement to act as a lift limb device. The U.S. Pat. No. 3,685,797 issued to B. E. Orr is an example of such a device. A problem inherent in many simple lever arrangements is that the lever in pivoting about a point causes the loaded or free end of the lever to define an arcuate path as it rises. Consequently, either the load must be movable to follow the arcuate path as it is raised, or the base of the jack must be movable to compensate for the arcuate path and the immovability of a load. In the first case, problems result because many loads are simply too large or are immovably attached at an opposite end so that they are not laterally movable. The second alternative may result in instability of the jack as the load is raised, particularly where the base is not on wheels or is on a surface which resists movement.
Another problem encountered in the use of jacks is that the range of lifting is quite limited by the mechanism, frequently requiring blocking the load and resetting the jack to achieve greater heights. The lever-type jacks have alleviated this problem to some extent; however, it has been found that in attempting to devise jacks with higher lift ranges, the resulting jack apparatus becomes either very bulky and unwieldy or relatively unstable, and in addition is not readily movable between different locations or collapsible into a compact configuration requiring a minimum of space for storage and transportation.
Several prior art patents which have attempted to accommodate some of these desirable features while eliminating the problems include: U.S. Pat. No. 3,582,043 issued to J. Tranchero; U.S. Pat. No. 3,378,231 issued to W. A. Rapp; U.S. Pat. No. 3,361,409 to M. R. Stahl; and U.S. Pat. No. 3,685,797 issued to B. E. Orr. While these prior art patents have been successful to some degree, the present invention is a significant improvement over the prior art not only in lifting range, lifting capacity, and stability, but also in convenience moving, setting and operating, reliability, and compactness for storage and transportation.